The invention relates to a method for the detection and/or determination in a test sample of a component of the reaction between a specifically binding substance and a corresponding bindable substance, using the mutual reactivity of such components and of at least one labelled component, and detecting the label, optionally after a separation of free and bound labelled component, in the test sample or in one of the fractions obtained after separation.
Such a process is known from many recent publications, such as European patent applications EP-A-0 007 654 (Akzo N.V., publ. Feb. 6, 1980) and EP-A-0 158 746 (Janssen Pharmaceutica N.V., publ. Oct. 23, 1985) which disclose the use of a labelled component comprising sol particles of a metal or metal compound, in particular gold and silver sol particles, as the label. In European patent application EP-A-0 032 270 (Akzo N.V., published Jul. 22, 1981) the utility as a label of sol particles of a hydrophobic dye or pigment is disclosed. The use as a label of sol particles of non-metallic elements, such as selenium ($e), tellurium (Te) and sulfur (S) is disclosed in European patent application EP-A-0 298 368 (Abbott Laboratories, publ. Jan. 1, 1989) and in Dutch patent application NL-A-87.02769 and the corresponding part of EP-A-0 321 008 (in the name of H.B.T. Holland Biotechnology B.V., published Jun. 21, 1989).
It is suggested in the above mentioned EP-A-0 158 746 that the detection of colloidal metal particles bound at the surface of an immobilized matrix, e.g. the nitrocellulose strip used in a dipstick assay, can be improved by increasing their visibility as a result of a physical development. Said physical development is obtained by the application of a physical developer which comprises a silver-containing compound, such as silver lactate or silver nitrate, which can be reduced to metallic silver. The physical developer further comprises a reducing agent, a buffer and stabilizing agents (see Swinburne, Immunocytochemistry, 1987, 30-37). The reaction initially takes place at the surface of the metal particles which catalyze the reduction, and then, subsequently, becomes auto-catalytic on these seeds. A black, highly contrasting signal is formed due to the accumulation of metallic silver. A considerable increase in sensitivity results therefrom.